The present invention relates to a monolithic integrated circuit containing an analog to digital converter (which will be shortly referred to as the "A/D converter") for forming at least a part of a digital control system.
A digital control system can have its size reduced by using a semiconductor integrated circuit. Moreover, the digital control system can be made to exhibit a relatively high reliability by being enabled to reduce the number of the external wires of the semiconductor integrated circuit.
In order to make a system control of high quality possible, a control system such as a process control system (making use of a microprocessor) or a computing control system is made receptive of not only the signals of various kinds of sensors (which will be referred to as the "digital sensors") made operative of digital or pulse signals but also analog signals bearing much information through the A/D converter.
In this case, both an input circuit (i.e., a digital input circuit) for feeding the data signal from a digital sensor to the data bus of a microprocessor and an input circuit (i.e., an analog input circuit) for feeding the data signal from an analog sensor to the data bus after it has been converted into a digital signal are desired to be constructed of a one-chip monolithic semiconductor integrated circuit (which will be shortly referred to as the "monolithic IC" or "IC") in order partly to reduce the size of a device constructing the system and partly to reduce the number of the external wirings of that device, as has been described in the above.
Moreover, the monolithic IC is desired to be so constructed that is can be applied to a variety of control systems.
By affording the general usability of the monolithic IC, this monolithic IC can be used in a variety of different control systems so that the maintenance of the various control systems can be facilitated. Moreover, the change and development of the control systems can also be facilitated.
In the usual monolithic IC, however, it should be noted that the number of the external terminals to be attached to the package thereof is limited by the size of that package, for example.
In case the number of external terminals to be attached to the IC package is limited so that the numbers of both analog and digital input terminals are accordingly limited, the monolithic IC has its applicability limited to a restricted control system.
For instance, the monolithic IC, which has its digital input terminals relatively reduced by the provision of a relatively great number of the analog input terminals, is difficult to use in a control system having a relatively great number of digital input signals. On the contrary, the monolithic IC, which has its analog input terminals relatively reduced by the provisions of a relatively great number of the digital input terminals, is difficult to use in the control system which is fed with a relatively great number of analog input signals.
The monolithic IC having the construction thus far described makes it difficult to change the control systems.
For examples, the output data signal from the analog sensor has its level made proportional to the physical quantity to be detected so that it can bear more information than that output data signal from the digital sensor, which is made to correspond to whether the physical quantity to be detected exceeds a certain threshold level or not. Therefore, the control system developed in advance can be easily changed, if the digital sensor can be replaced by the analog sensor, into such a control system as can perform a system control of higher quality. However, in case the monolithic IC having its digital input terminals relatively increased is used, the system change becomes difficult because the analog input terminals are relatively reduced.
In case the monolithic IC is to be equipped with the number of analog and digital input terminals required, the size of its package has to be enlarged because of the increase in the number of external terminals.